Exploratory Syntheses of New Alkali Transition Metal Based Chalcophophates & Halides

Das, Manajit and Jai, Prakash (2019) Exploratory Syntheses of New Alkali Transition Metal Based Chalcophophates & Halides. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

The transition metal thiophosphates are well known for their intricate crystal chemistry due to stabilization of various type of transition metal polyhedra and their sharing with PxQy units. The tendency of transition metals to show variable oxidation states coupled with the formation of various types of PxQy units help these compounds to adopt diverse structure types ranging from simple three-dimensional (3D) to complex layered (2D) and one-dimensional (1D) structures. Hence, this fascinating class of compounds exhibits a wide range of physical properties such as nonlinear optical properties, semiconducting properties, long-range magnetic ordering, etc. The incipient ionic conductivity of these compounds has caused re-emergence of this field as evidenced by the numerous recent articles appeared in the literature. Thus, we have focused our attention on the exploratory synthesis of new metal thiophosphates to extend and explore their physical properties. Our exploration attempts have been successful in synthesizing a new quaternary metal thiophosphate Cs2FeP2S6. The single crystals of this compound have been obtained by the reaction of iron powder and P2S5 with an excess of CsCl that acts as a reactant as well as a molten flux. This quaternary compound has been characterized by the single crystal X-ray diffraction technique. It crystallises in the centrosymmetric P21/n space group of the monoclinic crystal system with four formula units. The unit cell of this structure has dimensions of a = 6.0308(6), b = 12.715(1), c = 7.7020(9) Å and β = 93.282(4)o. Energy dispersive X-ray spectroscopic analysis of the selected single crystals shows the presence of the elements in the ratio as Cs:Fe:P:S ≈ 2:1:2:6. The band gap of this compound is further calculated by ultraviolet visible reflectance spectroscopy. It has a direct band gap of 2.3(2) eV with a very low indirect bad gap of 1.1(2) eV. The crystal structure of Cs2FeP2S6 can be described as consisting of one dimensional chains of ■(1@∞)[FeP2S6]2– that are propagating along the [100] direction, and these chains are separated by Cs atoms. We have also obtained a new mixed metal halide, Cs3FeCl5, by exploratory synthesis. The single crystal X-ray diffraction study shows that it crystallizes in the centrosymmetric I4/mcm space group of the tetragonal crystal system with unit cell dimensions of a = b = 9.279(1) Å and c = 14.824(3) Å. The crystal structure of this compound can be described as a pseudo zero dimensional structure with sixteen formula units. Each Fe atom in this structure is coordinated to four chlorine atoms in a tetrahedral fashion, and these isolated FeCl42– units are separated by Cs+ cations.

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